Associations among ribosomal (r)DNA intergenic spacer length, growth rate, and C:N:P stoichiometry in the genus Daphnia

Abstract

The growth rate hypothesis (GRH) proposes that variation in organismal C : N : P ratios is associated with growth rate due to differential allocation to P-rich ribosomal RNA to support growth. A further extension of the GRH proposes that this variation is associated with the structure of ribosomal (r)DNA genes (e.g., promoter-enhancer sequences in the rDNA intergenic spacer [IGS]) that influence the production rate of rRNA. To test these ideas, we examined growth rate, rDNA structure, RNA : DNA ratios, and elemental composition (C : N : P) in multiple clones of three Daphnia species (pulex, pulicaria, magna) in controlled laboratory experiments. Results show that a portion of the rDNA motif, consisting primarily of the IGS, shows considerable interspecific and intraspecific length variation. Significant interspecific differences in all parameters were found, while significant intraspecific (clone) effects were detected only for RNA : DNA ratios and growth rate. As predicted by the GRH, there were significant positive correlations between RNA : DNA ratio and either growth rate or rDNA/IGS length, and a significant positive correlation between rDNA/IGS length and growth rate, when clonal means for all three species were examined. However, no clear-cut relationships between RNA : DNA and either C : P or N : P were observed for any of the three species. Likewise, no discernible intraspecific relationships between growth rate and either C : P or N : P were detected. These preliminary findings suggest that further study of elemental composition, nucleic acid content, and growth rates of organisms may lead to a clearer understanding of the role of stoichiometric constraints in influencing the ecology and evolution of natural populations.

Patterns of Intraspecific DNA Variation in the Daphnia Nuclear Genome

“…Numerous studies have outlined physiological and life history differences between North American D. pulex and D. pulicaria (e.g., Brandlova et al 1972;Tessier and Consolatti 1991;Dudycha 2004;Weider et al 2004). In some parts of their distribution, D. pulex and D. pulicaria switch to sexual reproduction in response to different photoperiodic cues, resulting in a prereproductive isolating barrier (Deng 1997), and they usually occupy different but geographically overlapping habitats.…”

Growth rate, protein:RNA ratio and stoichiometric homeostasis of submerged macrophytes under eutrophication stress

“…Previous studies have applied the slope of body P and RNA-P to estimate the P allocation. If the slope is less than 1, the P is mainly used for the synthesis of RNA; If the slope is more than 1, the P is mainly used for the synthesis of organic compounds, such as nucleoptides cursors Matzek and Vitousek, 2009;Weider et al, 2004). In our study, we applied the slope of tissue N:P and protein:RNA to estimate the N and P allocation in submerged macrophytes.…”

Linking genes to communities and ecosystems: Daphnia as an ecogenomic model

“…P requirements for Daphnia vary genetically among populations, with greater P demand in animals from habitats with shorter growing seasons, and hence higher growth rates [27]. In the laboratory, selection for rapid growth results in a heritable response [28], and both the structure of ribosomal rDNA [29] and specific alleles of the phosphoglucose isomerase (Pgi) locus [30] have been associated with variation in growth rates and P demand among Daphnia clones.…”